Technical Field
The invention relates to a method and to apparatus for studying the properties of a multiphase fluid under pressure flowing along a duct and containing at least two liquid phases and one gaseous phase. The method and the apparatus of the invention make it possible, in particular, to determine the volume ratio of the liquid phases and to perform other measurements, such as density measurements, on each of the liquid phases.
A particular application of the invention lies in studying the properties of a petroleum stream flowing along a surface duct connected to one or more oil wells.
Background Information
Exact knowledge of the volume or mass ratios of the various fluid phases taken from an oil well is essential for operating the well.
In practice, such knowledge is difficult to acquire because oil well effluents generally comprise two liquid phases, constituted by water and liquid petroleum, plus a gas phase constituted by gaseous hydrocarbons, frequently in association with a solid phase constituted by sediment, sand, etc.
The volume or mass ratios of the various phases are frequently determined from a sample taken from the multiphase fluid under pressure flowing in the duct concerned. The most accurate technique for performing such sample-taking consists in enclosing a flowing fluid sample between two quick-closing valves placed directly in the duct or in a duct connected in parallel therewith. That technique has the advantage of enabling global measurement to be performed over the entire section of the duct concerned. However it depends on the speed with which the valves can close, and the number of valves concerned must be doubled when a parallel duct is provided, which is essential if it is desired that the flow of fluid should continue while a sample is being taken.
Such valve closure speed requires high control power and gives rise to high cost. In addition, the use of valves having moving parts constitutes a clear handicap from the point of view of reliability. Finally, it should be observed that this technique is unsuitable for performing accurate analysis of the fluid when the gaseous phase predominates to a large extent (e.g. constituting greater than 90% by volume). The quantity of liquid taken is then insufficient to enable accurate measurements to be performed. For all these reasons, the above technique is not used.
Given the difficulties raised by the technique of using quick-closing valves a simpler technique has been developed for the purpose of taking a plurality of small-volume samples in succession and accumulating the liquid phases of said successive samples in a common tank. That technique is illustrated in particular by U.S. Pat. Nos. 4,147,062 and 4,262,533, and it makes it possible to determine over a certain period, the mean volume ratio of the liquid phases contained in the fluid under pressure flowing along the duct. The samples are taken by means of a piston driven with reciprocating motion in a cylinder that opens out radially into the duct concerned. The two ends of the piston are in sealing relationship with the cylinder while the central portion of the piston has an annular recess. In the advanced position of the piston, its annular recess is located in the duct and is filled with fluid. When the piston is retracted into the cylinder, a small quantity of fluid is captured in the annular recess of the piston. The liquid phases of the fluid then fall under gravity into a tank while the gaseous phase is conventionally returned to the duct.
Although that piston sample-taking technique is simpler to implement than the quick-closing valve technique, it too suffers from the drawback of requiring the presence of moving parts that can give rise to breakdown. U.S. Pat. No. 4,776,210 proposes another sample-taking technique. In that case, the duct along which the multiphase fluid under pressure flows includes a horizontal portion in which fluid flow is reversed. More precisely, the fluid arrives in a given direction via a first horizontal duct that opens out facing a vertical wall. After striking the wall, the fluid rebounds in the opposite direction into a second horizontal duct disposed either around the first or to one side of it.
In accordance with U.S. Pat. No. 4,776,210, a small fraction of fluid is taken via at least one tube opening out in line with the end of the first duct. The other end of the tube is connected to a centrifugal separator whose top communicates via a second tube with a length of the duct situated downstream from the fluid flow reversal system, to return the gas phase thereto. The liquid phases collected in the bottom of the centrifugal separator are returned to the duct but significantly further downstream, after passing through an electromagnetic radiation measuring apparatus for determining the fraction of water present in the liquid phases.
Although the technique described in U.S. Pat. No. 4,776,210 makes it possible to eliminate moving parts, it suffers from the significant drawback of greatly disturbing the flow of fluid and of requiring significant modification to the length of duct from which the sample is taken.
It should also be noted that the accuracy with which the properties of a multiphase fluid are studied, e.g. the volume ratio of the liquid phases it contains, depends on the representativeness of the sample taken. The representativeness of the sample taken depends in particular on preserving thermodynamic equilibrium, i.e. on maintaining pressure and temperature during sample-taking and throughout subsequent operations. Unfortunately, maintaining thermodynamic equilibrium, and in particular temperature, is not described in the prior art.